The present invention relates to optical fiber transmission systems, and more specifically to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
The index profile of optical fibers is generally characterized as a function of the shape of the graph of the function which associates the refractive index and the radius of the fiber. It is conventional to plot the distance r from the center of the fiber on the abscissa axis and the difference between the refractive index and the refractive index of the cladding of the fiber on the ordinate axis. The expressions xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d and xe2x80x9ctrianglexe2x80x9d are used for the index profiles of graphs which are respectively step-shaped, trapezium-shaped and triangular. These curves are generally representative of the theoretical or set point profile of the fiber and fiber fabrication constraints can yield a significantly different profile.
It is advantageous to control chromatic dispersion in new wavelength division multiplex (WDM) transmission networks using high bit rates, especially bit rates of 10 gigabits per second (Gbit/s) and above; the objective is to obtain substantially zero cumulative chromatic dispersion over the whole of the link for all wavelengths of the multiplex, in order to limit broadening of the pulses. A cumulative dispersion value of a few hundred ps/nm over the whole of a transmission system is acceptable. It is also useful to avoid zero values of chromatic dispersion in the vicinity of wavelengths used in the system, since non-linear effects are more accentuated at such zero values. Finally, it is also useful to limit the chromatic dispersion slope over the range of the multiplex in order to prevent or limit distortion between channels of the multiplex. This problem of compensating chromatic dispersion and chromatic dispersion slope is particularly acute in transmission systems using very high bit rates, typically WDM transmission systems using a bit rate per channel of 40 Gbit/s and above. The problem becomes increasingly acute as bandwidth increases and reaches values as high as or greater than 30 nanometers (nm) or even 35 nm.
Single-mode fiber (SMF) is conventionally used as line fiber in optical fiber transmission systems. The applicant""s ASMF 200 single-mode fiber has a chromatic dispersion cancellation wavelength xcex0 in the range 1300 nm to 1320 nm and chromatic dispersion that is less than or equal to 3.5 picoseconds per nanometer kilometer (ps/(nm.km)) in the range 1285 nm to 1330 nm and that is equal to 17 ps/(nm.km) at 1550 nm. The chromatic dispersion slope at 1550 nm is of the order of 0.06 ps/(nm2.km).
Dispersion-shifted fibers (DSF) are now available off the shelf. They have substantially zero chromatic dispersion at the transmission wavelengths at which they are used, which as a general rule are not the wavelength of 1.3 micrometers (xcexcm) at which the dispersion of silica is substantially zero; this means that the non-zero chromatic dispersion of the silica is compensated by an increase in the index difference xcex94n between the core of the fiber and the optical cladding. This explains the use of the term xe2x80x9cshiftedxe2x80x9d: the index difference shifts the wavelength at which there is zero chromatic dispersion, and is obtained by introducing dopants into the preform during fabrication thereof, for example by a modified chemical vapor deposition (MCVD) process that is known to the person skilled in the art and is not described in more detail here.
At the wavelengths at which they are used, non-zero dispersion-shifted fibers (NZxe2x88x92DSF+) have low non-zero positive chromatic dispersion, typically less than 10 ps/(nm.km) at 1550 nm, and chromatic dispersion slope in the range 0.04 ps/(nm2.km) to 0.1 ps/(nm2.km).
FR-A-2 790 107 proposes a line fiber which is particularly suitable for dense WDM transmission with a channel spacing of 100 GHz or less for a bit rate per channel of 10 Gbit/s; at a wavelength of 1550 nm, this fiber has an effective surface area greater than or equal to 60 xcexcm2, a chromatic dispersion in the range 6 ps/(nm.km) to 10 ps/(nm.km) and a chromatic dispersion slope less than 0.07 ps/(nm2.km).
Using short lengths of dispersion-compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in single-mode line fiber or non-zero dispersion-shifted line fiber is known in the art. An example of a transmission system in which chromatic dispersion in a single-mode line fiber is compensated by using dispersion-compensating fiber is described by M. Nishimura et al. in xe2x80x9cDispersion-compensating fibers and their applicationsxe2x80x9d, OFC""96 Technical Digest ThA1. The use of a dispersion-compensating fiber is also mentioned by L. Grxc3xcner-Nielsen et al. in xe2x80x9cLarge volume manufacturing of dispersion-compensating fibersxe2x80x9d, OFC""98 Technical Digest TuD5. A drawback of this type of fiber is its high cost.
Dispersion-compensating fibers are described in a number of patents. At wavelengths in the vicinity of 1550 nm they have a negative chromatic dispersion, which can be used to compensate the cumulative chromatic dispersion in the line fiber, and can also have a negative chromatic dispersion slope, which can be used to compensate the positive chromatic dispersion slope of the line fiber. U.S. Pat. Nos. 5,568,583 and 5,361,319 propose a dispersion-compensating fiber suitable for compensating chromatic dispersion in single-mode fiber and having a chromatic dispersion of the order of 17 ps/(nm.km) at a wavelength of 1550 nm. WO-A-99/13366, EP-A-0 674 193 and U.S. Pat. No. 5,838,867 provide other examples of dispersion-compensating fibers for use with dispersion-shifted fibers. The drawbacks of dispersion-shifted fibers are their cost and the attenuation that they introduce into the system.
Dispersion-managed fibers (DMF) having dispersions that vary with length have been proposed. These fibers are an alternative to using dispersion-compensating fibers. One proposed solution forms a fiber with adjacent sections having opposite chromatic dispersions and chromatic dispersion slopes, with transition regions between sections that are as short as possible. For example, fibers of this kind are proposed in EP-A-0 737 873, EP-A-0 949 520, EP-A-0 949 519, WO-A-99/57822, WO-A-99/42869 and U.S. Pat. No. 5,887,105. They limit non-linear effects, the chromatic dispersion remaining high except in the short transition regions; the total chromatic dispersion of the fiber is controlled by choosing the lengths and chromatic dispersions of the sections.
In xe2x80x9cDesigning a large effective area fiber for submarine systemsxe2x80x9d (NFOEC""99, National Fiber Optics Engineer Conference), T. J. Atwood and W. K. Adcox highlight the importance of the effective surface area in reducing non-linear effects; they indicate that the effective length of non-linear interactions is of the order of 20 kilometers (km) for single-mode fiber. They also propose an ideal dispersion profile for a transmission system.
Making the chromatic dispersion of fiber for transmitting RZ soliton signals decrease exponentially as a function of the length of the fiber has also been proposed; the resulting fibers are called dispersion-decreasing fibers (DDF). The chromatic dispersion varies along the fiber to preserve soliton conditions for propagation along the fiber, despite the attenuation of the signals. Thus EP-A-0 789 256 proposes a fiber in which the index profile is progressively varied between the two ends of the fiber. To enable soliton transmission, the fiber has positive chromatic dispersion at one end and zero or very low chromatic dispersion at the other end. The fiber is obtained by fabricating a preform with a varying profile, which yields a fiber with a constant diameter after drawing. The fiber is used as line fiber in soliton signal transmission systems. EP-A-0 789 256 also proposes a decreasing-dispersion fiber for soliton transmission. That document proposes the use of discrete chromatic dispersion changes along the fiber, to approximate an exponential decrease in the chromatic dispersion of the fiber, rather than varying the characteristics of the fiber continuously. WO-A-98/25861 proposes a method of fabricating this kind of fiber.
With the objective of reducing backscattering by the Brillouin effect, even if the intensity of the light injected into the fiber increases, EP-A-0 518 749 proposes a stepped index fiber having varying propagation characteristics. It proposes to vary the diameter and the refractive index of the core, the composition of the glass and the residual tension in the core along the length of the fiber accordingly. The variations proposed in the examples are periodic variations.
The decreasing-dispersion fibers proposed in the prior art, and in the last-mentioned document in particular, are suitable for soliton signals. Against the background of the prior art discussed above, the problem addressed by the invention is that of compensating chromatic dispersion in transmission systems, and in particular in terrestrial transmission systems for non-soliton signals. The invention proposes a solution for limiting the total chromatic dispersion in the transmission system and thus the length of dispersion compensating fiber needed. Another problem that arises in the prior art, and which is solved by some embodiments of the invention, is that of fabricating fibers whose propagation characteristics vary with length; the solution proposed in WO-A-98/25861 entails compensating a conical configuration preform, which complicates the fabrication process.
To this end, the invention proposes a fiber whose chromatic dispersion varies as a function of length. The fiber has higher chromatic dispersion at its first end than at its second end. Moreover, the chromatic dispersion of the fiber decreases in a regular or localized manner over at least a portion of the length of the fiber.
The fiber is used in particular in a transmission system as the line fiber at the start of a section between two repeaters; the value of the chromatic dispersion in the vicinity of the first end of the fiber limits non-linear effects when the signals transmitted in the fiber still have high power. The decrease in chromatic dispersion enables a transition to a line fiber of lower chromatic dispersion, which can moreover form part of the fiber. The combination has cumulative chromatic dispersion lower than the prior art line fiber, which limits the need for chromatic dispersion compensation.
To be more precise, the invention proposes an optical fiber having at its first end chromatic dispersion in the range +6 ps/(nm.km) to +17 ps/(nm.km) and at its second end a chromatic dispersion in the range +3 ps/(nm.km) to +14 ps/(nm.km); the difference between the chromatic dispersion at the first end of the fiber and the chromatic dispersion at the second end of the fiber is greater than or equal to 2.5 ps/(nm.km) and chromatic dispersion varies continuously along the fiber.
The fiber preferably has a first portion in the vicinity of its first end in which the average chromatic dispersion is greater than or equal to 6 ps/(nm.km). The first part can have a length greater than or equal to 10 km; the rate of longitudinal variation of the chromatic dispersion in the first portion is preferably in the range xe2x88x921.4 ps/(nm.km2) to +0.1 ps/(nm.km2).
In one embodiment of the invention the rate of variation of chromatic dispersion along the fiber is substantially constant. This rate can have a value in the range xe2x88x921.4 ps/(nm.km2) to xe2x88x920.05 ps/(nm.km2).
The variation of chromatic dispersion can be effected substantially completely within a length of the fiber less than the total length of the fiber. In a first example, the length of the portion of the fiber in which the chromatic dispersion decreases is in the range 10 km to 35 km. In this case, the rate of longitudinal variation of the chromatic dispersion in the portion of the fiber in which the chromatic dispersion decreases is preferably in the range xe2x88x921.4 ps/(nm.km2) to xe2x88x920.07 ps/(nm.km2).
The length of the portion of the fiber in which the chromatic dispersion decreases can also be in the range 1 km to 5 km. In this case, the rate of longitudinal variation of the chromatic dispersion in the portion of the fiber in which the chromatic dispersion decreases is advantageously in the range xe2x88x9214 ps/(nm.km2) to xe2x88x920.5 ps/(nm.km2).
In all case, the fiber can have a second portion in the vicinity of its second end in which the chromatic dispersion is less than or equal to 14 ps/(nm.km). The second portion can have a length greater than or equal to 1 km.
The fiber advantageously has at any point an index profile in the shape of a rectangle with a depleted trench and a ring. In this case, the radii of the rectangle, the depleted trench and the ring are preferably identical along the fiber.
The invention also proposes a transmission system including at least one repeater and a fiber of the above kind receiving at its first end signals amplified in the repeater. A fiber with substantially constant chromatic dispersion can be connected to the second end of said fiber.